The insulin and insulin-like growth factors, which have many actions in the placenta, may be derived from the fetal circulation, the maternal circulation, and/or the trophoblastic cells. Their activities are potentiated by transport proteins through receptors they activate. The binding of 125I-IGF-II to the two purified membranes supports the notion that the membranes are polarized for these receptors (Fig. 3). This type of differential distribution has also been reported for another cytokine, epidermal growth factor, for which there were found to be 40% more receptors on microvillous membranes than on basal plasma membrane. Buy Asthma Inhalers Online
Similarly, there are 3.5 times more endothelin-1-binding sites on the basal plasma membrane than on microvillous membrane. The distributions of subfamilies of protein kinase С and several G proteins, or the calcitonin gene-related peptide receptors, are also asymmetrical in human syncytiotrophoblast. The polarization can be even more extreme for such systems as the syncytial (3-adren-ergic receptors coupled to the adenylate cyclase system, which are present only on the basal plasma membrane; the opioid kappa-type receptors are present only on the microvillous membrane.